1. Field of the Invention
This invention refers to a horizontally oriented scanning head for numerically controlled gear-measuring instruments having a scanning pin or probe carrier which can be deflected parallel to itself with several degrees of freedom, which is linked to a stationary scanning head base via several spring-parallelogram systems in functionally parallel arrangement, and whose deflections are detectable by three sensors attached to the scanning head base for the three spatial coordinate directions X, Y, and Z. The horizontal Y-direction coincides with the longitudinal axis of the scanning head, the X-direction corresponds to the lateral deflection of the scanning pin carrier and the Z-direction to its vertical deflection.
2. Description of Related Art
Such a scanning head is described in DE (German patent document) 3905952 A1, and its structural embodiment is shown in FIG. 6 of this patent publication. Between the stationary and the movable parts of the scanning head, there are provided three linkages, or hinged systems connected in parallel, each comprising a spring hinge and two double-jointed rods. Together, they have the shape of a cube, with the stationary part of the scanning head forming three edges converging at one corner and the movable part forming three edges converging at the diagonally opposite corner of the cube, where the scanning pin is also located. With respect to the scanning forces at the scanning pin, the design is therefore asymmetrical. As a result, the torsionally rigid construction which is desired is not adequate for use in a gear-measuring instrument.
Another scanning head known from DE (German patent document) 19500451 A1 is designed symmetrically with respect to its central axis A, and its movable parts should have as small a mass as possible. The leaf spring hinges are arranged to permit the scanning pin carrier to be displaced in the direction of the central axis and to be pivotally mounted perpendicularly to the central axis with two degrees of freedom. However, this latter feature has the decisive disadvantage that the measured result depends on the instantaneous distance between the point of contact of the scanning head tip and the pivot point of the scanning pin. Therefore, this scanning head can not be used for gear-tooth measurements in which the scanning pin must also be interchangeable with longer or even angled scanning pins, without complicated calibration. This requirement can only be met by a scanning head with a scanning pin carrier which can be deflected parallel to itself.
In addition, vertically oriented scanning heads for coordinate measuring devices are known, as described, for example, in EP (European patent document) 0693669 A2 and DE (German patent document) 3412302 A1, in which spring-jointed parallelograms, one for each of the three coordinate directions, are suspended below each other. Due to the serial arrangement of their spring elements, these kinds of scanning head have the disadvantage that the mass which is moved when the uppermost parallelogram is deflected in one coordinate direction is much greater than when the lowest parallelogram is deflected in the other coordinate direction. Thus, the deflection forces and also the oscillatory behavior within these scanning heads vary greatly during the course of the measurement, in spite of equal spring forces but different masses. This has especially negative effects on the scanning of curved designs, for example, if the scanning head is guided along the specified curve and deviations in unknown directions are to be detected. That is because the scanning pin has to deflect of its own accord in the specific direction which is currently normal to the contour being scanned. This is possible only for scanning heads with equal static and dynamic behavior in every arbitrary direction of deflection.
Furthermore, there is known from DE (patent document) 19501178 C2, a pivotable two-coordinate scanning head which was specially designed for use on numerically controlled measuring devices to measure gears and gear-like workpieces. The scanning pin is freely movable in one plane, and this measuring plane can be swiveled into preset angular positions around a horizontal axis lying in that plane. Thus, the desired normal direction must constantly be known at every measuring point in this so-called 2D-scanning head, in order to enable the measuring plane to be swiveled appropriately into place before the measurement is made. Therefore, this scanning head lacks the 3D-features that are sometimes also desired for the complete measurement of a workpiece, as required for instance in scanning unknown contours. Furthermore, off-center conditions of the scanning ball, occurring when the entire scanning head is pivoted during a measurement, have negative effects on the measurement result, so that it is not possible to use angled scanning pins with scanning balls outside the pivot axis.